KT Consortium. Foto: Thorkild Christensen

Sustainable process design with process intensification

Abstract

Increased need for sustainable practices has resulted in a push for research and development in emission reduction. This is particularly prevalent for carbon dioxide emissions. Carbon dioxide capture and utilization is a promising method of reducing emissions due to the ability of offset (economically) the cost of carbon capture. To achieve this, a 3-stage framework for the sustainable synthesis-design of carbon dioxide capture and utilization has been developed. This framework integrates a number of computer-aided tools, including a database, superstructure optimization interface (Super-O), analyses and simulation software. This framework has been applied to the design and analysis of carbon dioxide capture and utilization processes to produce various value-added chemical products, including methanol, dimethyl carbonate, dimethyl ether, succinic acid and acetic acid, via conversion.

Introduction

Carbon dioxide (CO2) is the most prevalent greenhouse gas constituting over 80% of greenhouse gas emissions. Methods to reduce the concentration of carbon dioxide in the atmosphere, including carbon dioxide capture, utilization and storage, are needed. Carbon dioxide capture and utilization is a promising method, in addition to carbon capture and storage, which removes the carbon dioxide from emission streams and reuses it or transforms it to commercial products. Superstructure optimization has been performed in other works to optimize the types of capture and determine supply chain for carbon capture and utilization. However, these contain fixed utilization scenarios or only consider sequestration; superstructure optimization has not been applied to determining the optimal utilization path considering chemical conversion. Therefore, a framework has been developed following a 3-Stage approach: (1) synthesis, (2) design, and (3) innovation for the sustainable design of carbon dioxide capture and utilization processes. This framework decomposes the problem in each stage, first using superstructure optimization to determine the optimal processing route, then performing rigorous simulation and analyses to determine areas for improvement, and finally implementing process intensification methods [1] to find innovative and more sustainable solutions. The production of methanol and dimethyl carbonate have been designed and analysed to show that conversion processes can be economically competitive and environmentally beneficially. This work presents the 3-Stage approach to propose sustainable processing routes for a class of valuable chemicals.

3-Stage Framework

The 3-Stage framework is shown in Figure 1. In this method, the problem is decomposed to enable the solution of complex and large problems. In each stage, the model complexity increases as the number of alternatives decreases. In Stage 1, the processing route is selected from a network of alternatives. This stage incorporates reaction path synthesis to find different reaction opportunities (currently over 100 reactions have been generated producing commercial chemical products), a database to store the information, and an interface, Super-O, to facilitate the synthesis. In this way, unique opportunities and products are explored. In Stage 2, the selected processing route is designed and analysed by using simulation software and sustainability (economic, environmental and LCA) analysis tools. From this stage, hot spots and areas for improvement are also generated. In Stage 3, the targets for improvement are used to develop novel and more sustainable design alternatives, including the use of process intensification. The framework can be started at any stage provided the input information. The framework also incorporates computer-aided methods and tools within the steps.

Results and Discussion

The developed framework has been applied to an illustrative case study. The objective is to highlight the application of the methodology including the necessary methods and tools. In addition, this case study shows the sustainability and innovative possibilities for carbon dioxide capture and utilisation. First, a network is developed targeting smaller carbon, hydrogen and oxygen containing compounds. Using the reaction path synthesis tool, a network of over 100 reactions is generated. Then, data is required. For this, an ontology-based database has been developed. The necessary data is then retrieved so that the network is defined by a mathematical model. Using superstructure optimization, this can be optimised. For a small superstructure for the production of methanol, dimethyl ether and dimethyl carbonate this has been performed. Additionally, logic-based screening has been used to narrow the list of alternatives for the case of methanol synthesis. Rather than perform mathematical optimization, this small list of alternatives is taken directly to Stage 2 for detailed design. For the case of dimethyl carbonate (DMC) synthesis, Stage 1 determines the optimal processing route to be via the synthesis of ethylene carbonate. This superstructure optimization provides the route along with the objective function value, in this case defined as the revenue minus the cost of materials. The resulting process, using carbon dioxide and propylene oxide to produce propylene carbonate and subsequently the synthesis of dimethyl carbonate by adding methanol. Then, this is analysed using sustainability metrics, economic assessment and life cycle assessment. These show that the process can be designed sustainably (reducing carbon dioxide by -0.09 kg CO2/kg DMC) and economically (competitive selling price of DMC). Finally, the downstream separation is targeted for improvement by process intensification [2]. Using reactive distillation, the overall process can be improved, reducing the costs and increasing the sustainability.

Conclusion

A framework for the sustainable synthesis-design of carbon dioxide capture and utilization processes has been developed. This framework adopts a 3-stage approach, comprising of: (1) synthesis, (2) design, and (3) innovation, which also incorporates computer-aided methods and tools. This framework has been applied to the design of conversion processes to value-added chemicals, including methanol, dimethyl ether and dimethyl carbonate. With improvements to technology and increasing research in carbon dioxide capture and utilisation, such processes can help reduce over 10% of all carbon dioxide emission and with changing demands even more.

Supervisors

Prof. Rafiqul Gani
Prof. John M. Woodley

PhD Study started: October 2014 to be completed: September 2017

To see figures and references for this text, please find the original in the Graduate Schools Yearbook 2016

To see figures and references for this PhD study, please download the Graduate School Yearbook.

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